Gum arabic production in Acacia senegal plantations in the Sudanian zone of Cameroon: Effects of climate, soil, tapping date and tree provenance

Gum exudate after tapping Acacia senegal in Northern Cameroon. Photograph R. Peltier.

Jean-Michel Harmand1

Mama Ntoupka2

Bertrand Mathieu1, 2

Clement Forkong Njiti2

Jean-Marie Tapsou2 Jean-Christophe Bois1, 2 Philippe Thaler1 Régis Peltier3 1_Cirad Umr Eco&Sols 2 place Viala, Bât. 12 34060 Montpellier France 2_Irad

Programme forêts et bois

Centre régional de recherche agricole BP 222, Maroua

Cameroun

3_Cirad

Upr Bsef

Biens et services des écosystèmes forestiers tropicaux

Campus de Baillarguet 34398 Montpellier Cedex 05 France

Gum arabic production

in Acacia senegal plantations

in the Sudanian zone of Cameroon:

Effects of climate, soil, tapping date

RÉSUMÉ

PRODUCTION DE GOMME ARABIQUE EN PLANTATIONS D’ACACIA SENEGAL EN ZONE SOUDANIENNE DU CAMEROUN : EFFET DU CLIMAT, DU SOL, DE LA DATE D’INCISION ET DE LA PROVENANCE DES ARBRES

La présente étude a été menée sur des plantations d’Acacia senegal (L.) Willd. installées en zone soudanienne du Came-roun entre les isohyètes 650 mm et 1 250 mm. L’étude concerne la crois-sance de l’espèce, le choix de la date d'incision et l’influence des facteurs cli-matiques et édaphiques sur la production de gomme arabique. Les plantations ont été réalisées de 1985 à 1989 et les essais de saignée de 1993 à 1998. L’espèce a présenté généralement une bonne adap-tation et une bonne croissance dans les différentes conditions de site de la zone d’étude. Les observations montrent une meilleure production de gomme quand la saignée a été réalisée en début de saison sèche, lorsque l’humidité relative dimi-nuait. Selon l’isohyète, la date optimale de saignée s’est étalée du 10 octobre (650 mm) au 25 novembre (1 250 mm). Entre 650 mm et 800 mm de pluviosité annuelle, la production moyenne annuelle de gomme dans chaque site a été de 100 g à 500 g par arbre saigné, correspondant à un rendement à la par-celle de 50 à 250 kg/ha/an pour une den-sité de 500 arbres/ha. Au-dessus de l’isohyète 1 000 mm, la production s’est montrée plus aléatoire. Bien qu’à l’échelle pluriannuelle la production de gomme ait été similaire dans les diffé-rents types de sols, le niveau de produc-tion annuelle a été plus variable sur sol sableux que sur sol argileux. La prove-nance soudanienne locale a été en géné-ral plus productive que toutes les prove-nances introduites d’origine sahélienne (Sénégal, Soudan) ou d’Inde. En plus de leur clarté remarquable, les quelques échantillons de gomme analysés de la provenance locale ont montré des pro-priétés classiques typiques des exsudats d’A. senegal de la ceinture sahélienne.

Mots-clés : date d'incision, facteur

éda-phique, facteur climatique, provenances d’Acacia senegal, agroforesterie.

ABSTRACT

SENEGAL PLANTATIONS IN THE

SUDANIAN ZONE OF CAMEROON: EFFECTS OF CLIMATE, SOIL, TAPPING DATE AND TREE PROVENANCE

This study was conducted in the Sudan-ian zone of Cameroon, where annual rainfall ranges from 650 to 1,250 mm, to assess the growth of Acacia senegal (L.) Willd. and evaluate the influence of tap-ping dates, as well as climatic and edaphic effects on gum arabic produc-tion. The tree plantations were estab-lished between 1985 and 1989, and the tapping tests were carried out between 1993 and 1998. Generally good adapta-tion and growth of the species were observed in the different site conditions of the study area. To optimize gum pro-duction, the best time to tap the trees was at the beginning of the dry season, when the relative humidity dropped. Depending on the location along the cli-matic gradient, the optimum tapping

date varied from October 10th_{(650 mm}

isohyet) to November 25th_(1,250 mm

isohyet). At 650 to 800 mm annual rain-fall, the average gum production per site varied from 100 to 500 g per tapped tree, corresponding to 50-250 kg/ha with a density of 500 trees/ha. However, in sites with annual rainfall higher than 1,000 mm, the gum production was gen-erally lower and uncertain. Although the mean production on the different types of soil did not differ significantly when years were combined, the annual pro-duction was more variable and more dependent on climatic variations on sandy soils than on clay soils. The local Cameroon Laf provenance was more pro-ductive than foreign Sahelian (Senegal, Sudan) or Indian provenances. First observations and analyses of gum sam-ples from the local provenance showed an outstanding brightness and classical properties typical of A. senegal exudates in the Sahel region.

Keywords: tapping date, edaphic factor,

climatic factor, provenances of Acacia Senegal, agroforestry.

RESUMEN

El presente estudio se llevó a cabo en plantaciones de Acacia senegal (L.) Willd. establecidas en la zona sudanesa de Camerún, entre las isoyetas de 650 mm y 1 250 mm. Este estudio se refiere al creci-miento de la especie, la elección de la fecha de incisión y la influencia de los fac-tores climáticos y edáficos en la produc-ción de goma arábiga. Las plantaciones se efectuaron de 1985 a 1989 y los ensayos de incisión de 1993 a 1998. La especie presentó generalmente una buena adap-tación y un buen crecimiento en las distin-tas condiciones de sitio del área de estu-dio. Las observaciones muestran una mejor producción de goma si las incisio-nes se realizan al inicio de la temporada seca, cuando disminuye la humedad rela-tiva. La fecha óptima para incisiones se determinó en función de la isoyeta entre el 10 de octubre (650 mm) y el 25 de noviem-bre ( 1 250 mm). Entre 650 mm y 800 mm de pluviosidad anual, la producción pro-medio anual de goma por sitio varió de 100 g a 500 g por árbol entallado , lo que corresponde a un rendimiento por parcela de 50 a 250 kg/ha/año con una densidad de 500 árboles por hectárea. En la zona que recibe más de 1 000 mm, la produc-ción mostró una mayor irregularidad. Aun-que a escala plurianual la producción de goma fue similar en los distintos tipos de suelo, el nivel de producción anual fue más variable en suelo arenoso que en suelo arcilloso. La procedencia sudanesa local se reveló, en general, más productiva que las procedencias introducidas de ori-gen saheliense (Senegal, Sudán) o de la India. Además de su notable claridad, las muestras de goma analizadas de la proce-dencia local mostraron propiedades clási-cas que caracterizan a los exudados de A. senegal en la región del Sahel. Palabras clave: fecha de incisión, factor

edáfico, factor climático, procedencias de Acacia Senegal, agroforestería.

Introduction

In the arid and semi-arid regions of Africa, land degra-dation and soil fertility depletion are considered to be the major threats to natural resource conservation and food security (Garrity et al., 2010). One of the potential solu-tions to land degradation is to promote the utilization, regeneration and planting of a native under-utilized legume tree: Acacia senegal (L.) Willd., the main species in the world producing the internationally traded arabic gum. A. senegal is a widespread woody species in the pastoral zones of Sahelian Africa. The larger natural stands are on deep sandy soils (fixed old dunes) in areas receiving from 300 to 500 mm annual rainfall (Dione, 1996). However, scattered A. senegal stands also occur on clay soils with an annual rainfall of 600 mm in Central and Eastern Sudan (Obeid, Seif El Din, 1970). In Northern Cameroon, A. sene-gal is common and mixed with other species in woodland savannas receiving up to 900 mm annual rainfall.

A. senegal shows promise as a multipurpose species for its range of products and uses: gum arabic, fodder and wood production, and soil fertility improvement (Raddad et al.,

2005). As an N₂-fixing species, A. senegal improves degraded

lands and nutrient deficient soils (Isaac et al., 2011). This species is particularly used in tree improved fallows to replenish soil fertility (Deans et al., 1999; Raddad et al., 2005), and intercrops well with sorghum and other grasses (Gaafar et al., 2006). Its ability to produce arabic gum (the true food-grade gum as it contains no toxins) is of particular economic interest which justifies a rural forestry based on plantations, following the example of the Sudanese agro-forestry farming system (Palou Madi et al., 2010). Neverthe-less, the success of A. senegal plantations in the Sudanian zone, which is wetter than its main zone of origin, will depend, for economic reasons, on the gum yields of the plan-tations (Harmand et al., 1997). In Burkina Faso, for example, natural and artificial stands show high yielding

potentials with up to 710 mm annual rainfall (Soloviev et al., 2010).

Ballal et al. (2005a) in the Sahelian zones of Sudan, and Dione (1996) in Senegal showed that gum production occurs only during the dry season, when the trees are shedding leaves. A threshold of water stress, consecutive to rain stoppage and dry air, seems required to trigger gum exudation (Dione, Vassal, 1998). Ballal et al. (2005a and b), and Raddad and Luukkanen (2006) showed a positive relationship between gum yield and rainfall in the season preceding tapping and/or between gum yield and soil water content at the end of the rainy season.

Ballal et al. (2005a) found that gum yield was positively correlated with tapping intensity, rainfall, and minimum and maximum tempera-tures at tapping time, and negatively correlated with tapping time, and minimum and maximum temperatures at gum collection. Late tapping reduced the production of gum.

In Northern Cameroon, experimental plantations of A. senegal have been created since 1985, in various pedocli-matic sites between 650 and 1,250 mm annual rainfall. The objective of the present study was to assess the growth of A. senegal and to evaluate the influence of the tapping date, as well as climatic and edaphic effects on gum production. Local provenance was compared with other Sahelian origins in terms of gum production and quality. This article synthe-sizes the results obtained on arabic gum production from 1993 to 1998 in order to answer the following questions: ▪ Can A. Senegal produce gum in the Sudanian Cameroonian zone, with 650 to 1,250 mm annual rainfall? The hypothesis being that a given level of water stress is required for gum exudation.

▪ Is it necessary to adapt the tapping calendar to the combi-nation of provenances and sites? The hypothesis being that leaf shedding and then the tapping date depend on both parameters.

▪ Is there an influence of the soil type on the survival rate, tree growth and gum production? The hypothesis being that the soil water regime resulting from the soil type will influ-ence tree growth and gum production.

▪ Can Sahelian provenances produce gum in Northern Cameroon? The hypothesis being that provenances are adapted to their zone of origin and may produce little gum in this wetter area.

Ultimately, answering these questions will determine the possibility of and provide information on developing sustainable and profitable gum production systems in the different areas of Northern Cameroon. However, fair prices can be offered for Cameroon gum only if it reaches quality standards. As no information was available on Cameroon gum, standard quality tests were performed and results are briefly presented in this article.

One of the potential solutions to land degradation is to promote the utilization, regeneration and planting of a native under-utilized legume tree: Acacia Senegal.

Materials and methods

The production of arabic gum was studied in seven sites in which A. senegal was planted between 1984 and 1989. Table I shows the characteristics of the sites and the plantations, all located in areas receiving an annual rainfall ranging from 650 to 1,250 mm (figure 1). There were three types of soils: sandy ferruginous soils (referred to as sandy soils), clay planosol “Hardé” (referred to as planosol) and clay vertisol. In the locations of Balda and Makalingay (fig-ure 1), the local provenance (Cameroon Laf), located 10 km south of Mouda, was compared to two provenances from Sahelian Sudan (Sudan North Kordofan and Sudan Blue Nile), two provenances from Northern Senegal (Senegal Diarno and Senegal Loumbé) and one provenance from India (table I). Sudan is the main arabic gum producing country, whereas Senegal provides the high quality Ferlo gum. India is an isolated area for A. senegal, with potential alternative genetic resources. The trees were planted with spacing of 4 m x 4 m (density of 625 trees/ha). The prove-nance comparison trials and two other trials on tree species screening (Aïssa Hardé and Salak) were set up in random-ized complete block designs with four repetitions. The dimensions of the plots were 28 m x 28 m (49 trees alto-gether, including 25 useful trees). The other trials (Mouda, Ngong and Touboro) consisted in mono-provenance plots, without experimental design. The survival rate of each plan-tation was evaluated and the height of the trees was meas-ured in each plot from 1985 to 1997.

Figure 1.

Experimental sites with Acacia senegal plantations in Northern Cameroon.

Table I.

Description of the different experimental sites and Acacia senegal plantations in Northern Cameroon.

Location Latitude and Altitude Soil Annual Origin of Annual rainfall Planting Survival rate

longitude (m) rainfall provenance in original site year (%)

(mm) of provenance 1994 1997

Aïssa Hardé 11°07’00’’N 325 Ferruginous 650 Cameroon Laf 800 mm 1984 80 45

14°19’45’’E (Sand dune) (Clay soil)

Balda 10°53’20’’N 330 Ferruginous 700 Sudan Blue Nile less than 600 mm 1985 84 83

14°38’00’’E (Sand dune) India Barmer Raj. 243 mm 34 28

Senegal Diarno 540 mm 60 60

Senegal Loumbé 550 mm 67 60

Cameroon Laf 800 mm 87 85

Makalingay 10°51’35’’N 416 Ferruginous 700 Sudan Blue Nile less than 600 mm 1989 73 68

14°15’00’’E (Sand dune) Sudan North Kordofan 365 mm 96 94

Cameroon Laf 800 mm 93 90

Salak 10°27’48’’N 430 Planosol ‘Hardé’ 800 Cameroon Laf 800 mm 1985 83 80

14°14’21’’E (Clay soil)

Mouda 10°22’48’’N 450 Vertisol 800 Cameroon Laf 800 mm 1984 85 69

14°13’43’’E (Clay soil)

Ngong 9° 02’ 09’’N 330 Ferruginous 1100 Cameroon Laf 800 mm 1989 100 100

13°30’38’’E (Sandy soil)

Touboro 7°46’29’’N 525 Ferruginous 1250 Cameroon Laf 800 mm 1986 100 100

Tapping treatments

The tapping trials were carried out from 1993 to 1998. Tapping, an essential treatment to induce gum exudation, was practiced with tools and the standard method used in Senegal. It consisted in removing 4-cm-wide and 30-cm-to-1-m-long strips of bark to expose the sapwood. Selected branches had a diameter greater than 5 cm and a smooth bark. Tapping was performed on two to three branches of each tree during the first four years (1994 to 1997), on all the branches during the pro-duction season 1997-98. After tapping, gum started to exude and formed nodules which were harvested and weighed every two weeks. The yield of each tree was cumulated over the har-vesting period, starting at the beginning of the dry season in October or November and ending in April. In 1997-98, two tap-ping dates were compared in all the locations but Touboro:

early tapping in October (from 10th_{to 21}st_{depending on the}

location, see table 3) and late tapping on November 10th_,

except in Ngong, where tapping was performed on

Decem-ber 18th_{. The Touboro plot was tapped once on November 25}th_.

In each situation (site, provenance) and for each year and date of tapping considered, 20 trees were randomly selected and tapped; in the complete block designs, five trees were selected in each of the four blocks.

In the production season 1996-97, three tapping dates were tested in Salak, and air humidity and temperature were obtained from a nearby meteorological synoptic station.

Analyses of variance were carried out to study the fac-tors provenance and date of tapping. In the case of a signifi-cant F-test, treatment means were compared using a Newman-Keuls HSD test. Gum production data were sub-jected to t-tests to analyze the differences between years and between sites within the local provenance (Cameroon Laf).

Gum samples of A. senegal were collected in 1998 from a few trees of Sudanese and local provenances, the lat-ter in two locations (Makalingay and Aïssa-Hardé) and included gum obtained from holes bored by insects. They were analyzed at the cellular Laboratory of Physiology of the University of Rouen in France. The analyses related to the traditional properties, i. e. viscosity, molecular mass, Lovibond colorimetric index and specific optical rotation.

Results and discussion

More than 80% of the planted trees from the local provenance (Cameroon Laf) survived after about 10 years in all the locations (table I). In Makalingay and Balda, survival rates of the Sudanese, Senegalese and Indian provenances were 68-94%, 60-67% and 28%, respectively (table I). The height growth of the local provenance ranged from 400 to 630 cm, 8 to 13 years after planting in the different sites (fig-ure 2). Hence, and compared to observations in Senegal (Deans et al., 1999) and Sudan (Raddad, Luukkanen, 2006), this growth was acceptable under a wide range of pedoclimatic conditions, with an annual rainfall from 650 to 1,250 mm, comprising sandy soils, and clay soils (vertisols and planosols “Hardé”) (figure 2). However, in the latter sit-uation, suitable rainwater harvesting measures such as small dykes and depressions were necessary to reduce sur-face runoff and thus to ensure a good start of the seedlings (Peltier, Eyog Matig, 1988; Harmand, 1993). After ten years, the higher survival rate and height growth observed in Touboro (annual rainfall of 1,250 mm) showed a good adap-tation of the species to sub-humid areas.

0 100 200 300 400 500 600 700 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997

Salak Touboro Aissa Hardé

Years Balda Ngong T re e h e ig h t (c m ) Figure 2.

Height growth of the local provenance (Cameroon Laf) of Acacia senegal in different locations of Northern Cameroon.

In the locations of Balda and Makalingay, the local provenance (Cameroon Laf: smooth bark on the left), was compared to two provenances from Sahelian Sudan (Sudan North Kordofan and Sudan Blue Nile: rough bark on the right). Photographs R. Peltier.

Seasonal dynamics of gum production

Gum exudation occurred approximately three weeks after tapping and continued up to April-May, at the arrival of the first rains. The peak of production occurred at vari-able dates. In Salak, considering the five years of produc-tion, maximum production was obtained at the end of December for early tapping (October), and in January for late tapping (November) (figure 3). This peak of production (g/tree/15 days) showed a positive linear correlation with the total annual production (g/tree). The simple regression model was significant (n= 6, P = 0.05; r² = 0.64):

Annual production = 4.69 x + 21.9

With annual production (g/tree), x peak of production (g peak/tree); data points were mean yield of the peak of production in relation to annual yield (average of 20 trees). Ballal et al. (2005a) proposed to use this type of model based on the yield of the first or second picking (Raddad, Luukkanen, 2006), and the date of tapping to predict the yield of the current year.

Gum production and climatic factors in the Sudanian zone of Cameroon

No significant variation was observed in the annual rainfall during the period 1990-2010 (figure 4): for example, for the periods 1993-98 and 2000-10 the average annual rainfall was 800 mm and 860 mm in Maroua-Salak (Far North Region), and 1,085 mm and 1,035 mm in Garoua (North Region), respectively. Therefore, we consider that the results obtained locally in 1993-98 are still valid for the 2000-2010 decade (figure 4).

Gum yield varied from 0 to 1,530 g/tree, and the max-imum was observed at Makalingay during the period 1997-98. Inter-tree variability of gum yield was high even within plots, regardless of the site or provenance (data not shown), similarly to previous results in other countries (Ballal et al., 2005; Raddad, Luukkanen, 2006; Jacques et al., 2010).

In the Far North Region of Cameroon (Balda, Makalingay, Salak and Mouda) with 650 to 800 mm annual rainfall (semi-arid area), the average gum production over 3 to 5 years of tapping was about 215 g per tapped tree (including the non productive trees), i. e. 110 kg/ha/y for a density of 500 trees/ha (table II). During the period 1997-98, the average gum yield of the local provenance varied from 170 to 500 g per tapped tree, i. e. from 85 to 250 kg/ha at the plot level, when tapping was carried out at the right time and with high intensity (see below) (table III). In the Senegalese Sahel (Ferlo area, with 250 to 400 mm annual rainfall), Dione (1996) reported average produc-tions varying from 45 to 350 g/tree accord-ing to sites, i. e. 11 to 90 kg/ha, as planta-tion densities were about 250 trees/ha. In Western Sudan (Northern Kordofan State, with 358 mm mean annual rainfall), Ballal et al. (2005a) reported during height years, average productions varying from 215 to 280 g/tree/y according to tap-ping dates, i. e. 86 to 112 kg/ha for a den-sity of 400 trees/ha. Comparatively, A. senegal seems a bit more productive in the Far North Region of Cameroon where rainfall varies from 650 to 800 mm than in the Sahelian zones of Senegal and Sudan where natural populations of A. senegal are more widespread. Therefore, gum pro-duction seemed linked to the occurrence of a pronounced dry season and not to a low annual rainfall. The higher water sup-ply occurring during the wet season in the Far North Region of Cameroon compared to Senegal and Sudan may be beneficial to gum production. A multi-local study in Burkina Faso (Soloviev et al., 2010) showed no clear relationship between gum production and annual rainfall, although the highest yield (225 g/tree) 0 20 40 60 80 100 120 16-No v 1-D ec 16-D ec 1-Jan 16-Jan 1-Feb 16-F eb 1-M ar 16-M ar 1-Apr 16-Apr 1-M ay 15-May 1993-94 1994-95 1995-96 1996-97 1997-98 1st tapping 19 1 1 19 1 1 19 1 1 19 1 1 19 1 1 1997-98 2nd tapping

Gum production (g tree

-1 15 d -1)

Harvesting date

Between November 1 and10 October 10 Tapping date : 400 600 800 1000 1200 1400 1600 1800 1989 1994 1999 2004 2009 Garoua Maroua-Salak Rainfall (mm y-1₎ Year Figure 3.

Gum production (g/tree) in the local provenance (Cameroon Laf) of Acacia senegal every 15 days according to the tapping date and the year. Data from the site of Salak (Far North Province of Cameroon).

Figure 4.

Annual rainfall (mm/yr)) during the period 1990-2010 in Maroua-Salak (Far North Cameroon) and Garoua (North Cameroon). Data from the synoptic stations of Maroua and Garoua airports.

was recorded in the location with the highest rainfall (710 mm per year) and the lowest (45 g/tree) in the driest location (565 mm). Moreover, Ballal et al. (2005b) showed in Sudan (North Kordofan) a positive correlation between annual gum yield over a six year period and annual rainfall received during the season preceding tapping. In the Senegalese Sahel, Dione (1996) also mentioned a positive correlation between annual rainfall and gum production over a seven year period. Also, in the Nigerian Sahel, Oleghe and Akinnifesi (1992) showed that an adequate water supply will help the produc-tion of gum in the dry season. In Senegal, Giffart (1973), Sene (1988) and Dione (1996) reported the need for signifi-cant rainfall during the rainy season, followed by a hot dry sea-son to support gum production.

However, the results showed average gum yields in Ngong (1,100 mm annual rainfall) generally much lower than those in drier sites, suggesting that above 1,000 mm annual rainfall (sub-humid area) gum production declines. Nevertheless, the first gum yield recorded in Touboro (1,250 mm) was comparable with that in sites of the Far North (table III). This is encouraging for the prospects of gum production in the Southern Sudanian zone.

Table II shows that gum yield in year 1997-98 was higher than in the previous years in almost all cases. The average temperature in November 1997 was higher (29°C) than those in November in the four previous years (26°C to 27°C). Such high temperatures at tapping time are known to be beneficial to gum exudation (Ballal et al., 2005a). However, the higher

Table II.

Mean gum production (g/tree/y) in the local (Cameroon Laf) provenance of Acacia senegal in seven locations in Northern Cameroon. Means within a line (campaign) followed by the same letter are not significantly different at P≤0.05 (n = 20).

Experimental site

Salak Mouda Makalingay Balda Aïssa Hardé Ngong Touboro

Soil Planosol Vertisol Ferruginous sandy soil

Age of plantation (y) 8-12 10-13 5-8 9-12 10-13 5-8 11

1993-94 218 - - - -1994-95 251 a - 127 b 123 b 186 ab 102 b -1995-96 157 a 124 a 20 b - - 51 b -1996-97 173 a - 185 a 227 a - - -1997-98 252 b 175 b 503 a 330 ab 391 a 21 c 210 b Mean 210 a 209 a 226 a 58 b Standard deviation 44 207 103 41 Table III.

Mean gum production (g/tree/y) in the local provenance (Cameroon Laf) of Acacia senegal according to the tapping date in seven locations in Northern Cameroon in the campaign 1997-98.

Means within a line (location) followed by the same letter are not significantly different at P≤0.05 (n = 20).

Tapping date 10-Oct 17-Oct 21-Oct 10-Nov 25-Nov 18-Dec P value

Aïssa Hardé 390.7 a 198.2 b 0.0002 Balda 206.4 a 330.1 a 0.06 Makalingay 503.2 a 393.8 a 0.7 Salak 252.2 a 186.5 a 0.06 Mouda 174.9 a 107.3 a 0.15 Ngong 21.7 a 13.5 a 0.4 Touboro 210

yield in 1997-98 could also be due to the fact that all tree branches with diameters larger than 5 cm were tapped, whereas in the previ-ous years, this was only the case for two to three branches per tree. However, Ballal et al. (2005b) reported that increasing the number of tapped branches per tree from 3 to 8 resulted in a low addition of gum yield.

Tapping date and gum production

During the production season 1996-97, three tapping dates were compared for the local provenance in Salak (figure 5A). The production was examined in relation to the relative humidity of the tapping period. Gum yield was significantly higher when tapping was carried out during the sharp decrease of the relative humidity (November 4) than when it was carried out before or after the change in air humidity. The higher efficiency of tapping carried out during the seasonal climate transition was confirmed by the higher percentage of producing trees.

For the production season 1997-98, table III shows the production obtained in each location at different tapping dates. The production was higher for trees tapped early (October), but the only significant difference between this production and that of the sec-ond tapping date (November 10) was in Aïssa Hardé. Ballal et al. (2005b) recorded that late tapping reduced gum yield by 40-50% in two different locations in North Kordofan. 0 20 40 60 80 100 0 50 100 150 200

10-Sep 20-Sep 30-Sep 10-Oct15-Oc t 20-Oc t 30-Oc t 4-No v 10-No v 20-No v 30-No v 6-D ec 10-D ec 20-D ec 30-D ec

10-Jan 20-Jan 30-Jan

R el ativ e humidity (%) Annual gum pr oduction (g tr ee-1) Annual production Relative humidity Tapping date

A: Expérimental site of Salak

0 20 40 60 80 100 0 50 100 150 200 10-Sep 20-Sep 30-Sep

10-Oct15-Oct20-Oct30-Oct4-No v 10-No v 20-N ov 30-N ov 6-D ec 10-D ec 20-D ec 30-D ec

10-Jan 20-Jan 30-Jan

R el ativ e humidity (%) Annual gum pr oduction (g tr ee-1) Annual production Relative humidity Tapping date

B: Expérimental site of Ngong Annual production R

R

Relative humidity A: Expérimental site of Salak

Annual production R

R

Relative humidity

B: Expérimental site of Ngong 60 Producing trees (%): 88 48 65 65 Producing trees (%):

More than 80% of the planted trees from the local provenance (Cameroon Laf) survived after 10 years in all the locations (Ngong on the left and Balda on the right). Photographs R. Peltier.

a

b

Figures 5.

Annual production of gum arabic in the local provenance (Cameroon-Laf) of Acacia senegal according to the tapping date in Salak (A) (Far North Province of Cameroon) and Ngong (B) (North Province of Cameroon).

Additional observations made in 1997-98 in Salak showed that trees had lost about 50% of their foliage when they were tapped for the first time (October 17), whereas the second tapping (November 10) resulting in less production corresponded to 80% defoliation. Consequently, the pheno-logical stage could constitute a good indicator for the choice of the tapping date. Considering the low accuracy of the visual estimation of the degree of defoliation, the level of 50% defo-liation is in accordance with Sene (1988) and Dione (1996) who mentioned that in Senegal higher gum exudations were obtained after tapping trees at two thirds of their defoliation. Low gum yield in Ngong (figure 5B) may similarly be related to too early (October 24) or too late (December 18) tapping. In this location, leaf flushes following late rains made the choice of the tapping date based on the phenological stage of trees difficult. In Touboro (1,250 mm annual rainfall) gum yield fol-lowing late tapping (at the end of November) was comparable with that in sites of the Far North Region (table III), which shows that managing the tapping date may be a key to ensure higher gum yield in sub-humid areas.

On the whole, as late tapping, after the drop in air humidity produced lower yields, our data are not consistent with the mere reference to a required threshold in water stress. There seems to be a limited favourable tapping time for gum production. Data collected could not indicate whether leaf fall was a mere indicator of favourable climatic conditions or a necessary physiological status to induce gum biosynthesis. However, other qualitative observations tended to indicate that defoliation seems to be necessary for gum exudation. In Salak (800 mm), two of the 80 trees tapped over four years did not produce gum. These trees

remained green later during the dry season. Similarly, tap-ping leafy branches in January did not induce exudation, whereas defoliated branches of the same trees, tapped at the beginning of the dry season, produced gum. Moreover, other observations showed that A. senegal trees planted in alluvial zones with shallow water table, neither defoliated at the beginning of the dry season, nor produced any gum ara-bic. Trials were not set up on these sites.

Another major observation on gum exudation was the effect of uncontrolled coleopteran (Bostrychidae) attacks at the end of the rainy season in Ngong (1,100 mm) in 1994 and Aïssa Hardé (650 mm) in 1997. Each time, these insects dug galleries in stems and branches causing early defoliation and profuse exudation from 250 g to 1 kg of gum per attacked tree per year. A 216 kg/ha production was obtained in Ngong in 1994-95 at the plot scale in response to insect attacks (Harmand, Bois, 1997). In Sudan, Jamal (1994) reported two species of Bostrychidae which bored galleries in the trunks of acacias: Apate monachus Boh. and Sinoxylon senegalense Karsch.

From a practical standpoint, indicative periods to carry out tappings can be proposed according to the geographical location, climatic conditions and phenological stage of the

trees (Maroua and North of Maroua: October 15th_–

Novem-ber 10th_{; Ngong, south of Garoua: November 10}th_–

Decem-ber 5th_).

Tappings carried out for five consecutive years did not seem to exhaust the trees, which indicated that exploitation might last over ten years, even twenty years as suggested by Muthana (1988) for Sudan.

Effect of uncontrolled coleopteran (Bostrychidae) attacks at the end of the rainy season in Ngong (1100 mm annual rainfall). These insects dug galleries in stems and branches causing early defoliation and profuse exudation from 250 g to 1 kg of gum per attacked tree per year. Photographs R. Peltier (left) and J.-M. Harmand (right).

Effect of soil type on gum production

Over the study period (all years confounded), mean annual gum production from the local provenance in the dif-ferent locations did not show any significant differences between sandy and clay soils. On the other hand, annual gum productions showed higher variations (higher standard deviation) on sandy soils than on clay soils (table II). Gaafar (2005) showed that gum production was positively correlated to soil water at 75-150 cm soil depth. Clay soils would thus guarantee a relatively stable production in spite of the interannual variability of climatic conditions, whereas, on ferruginous sandy soils, the level of production seems to depend on the seasonal rainfall pattern. This con-firms results by Séguiéri (1990) who observed in the same region that the development of the vegetation was more influenced by climatic variations on sandy soils than on clay soils as soil water content was more related to rainfall varia-tions on sandy soils.

Influence of tree provenance on gum yield

Imported provenances have been compared to the local one (Cameroon Laf) on the sites of Balda and Makalingay, both located in the Far North Region of Cameroon on sand dunes with 700 mm annual rainfall. The imported provenances origi-nated from drier areas of Senegal, Sudan and India.

At Balda (table IV-A), over the three-year monitoring period, the gum yield per tree was significantly higher in the local provenance than in imported provenances, except in 1997-98 when the Senegal Diarno provenance also per-formed well. Per area unit, the advantage of the local prove-nance was even greater, as it had the higher survival rate.

At Makalingay (table IV-B), the production of the local provenance was significantly higher than that of the Sudan Blue Nile provenance. However, gum yield of the second Sudan provenance, North Kordofan, was significantly lower only in 1996-97. Rainfall was low in 1995, which caused reductions in gum production in 1995-96 in the Cameroon and Sudan Blue Nile provenances, but not in the Sudan North Kordofan provenance. The latter seemed to adapt to lower rainfall and appeared less sensitive to the interannual variations of the climatic conditions. In 1997-98, the most productive tree was from Sudan North Kordofan (1,530 g), with a value higher than that of the most productive local tree (840 g). This provenance could thus include remarkable individuals that could be used in a breeding program (“plus trees”, Soloviev et al., 2010). Moreover, its survival rate was high in Makalingay.

On the whole, our results confirmed the hypothesis that in wetter areas, Sahelian provenances may not experi-ence the level of stress needed to produce high levels of gum. The local provenance performed better, both in terms of survival rate and gum production. Furthermore, according to Raddad and Luukkanen (2006) who showed that prove-nances from clay parts of the Sudanese gum belt were better adapted for fast growth, high biomass and gum pro-ductivity than provenances from the sand region, the clayey characteristics of the soil of Laf could also be in favor of the local Laf provenance.

Tables IV.

Mean gum production (g/tree/y) in different provenances of Acacia senegal in two experimental sites in the Far North Province of Cameroon. Means within a line (campaign) followed by the same letter are not significantly different at P≤0.05, according to Newman-Keuls HSD test (n = 4).

A – Experimental site of Balda

Campaign (tapping date) Provenance

Cameroon Senegal Senegal India Sudan P value

Laf Loumbé Diarno Blue Nile

1994-95 (2-Nov) 123 a 86 b 100 b 86 b 65 b 0.05

1996-97* (17-Oct and 10-Nov) 187 a 74 b 48 b 58 b 51 b 0.0001

1997-98* (21-Oct and 13-Nov) 268 a 60 b 309 a 141 b 97 b 0.0001

B – Experimental site of Makalingay

Campaign (tapping date) Provenance

Cameroon Sudan North Sudan Blue P value

Laf Kordofan Nile

1994-95 (1-Nov) 127 a 56 ab 21 b 0.04

1995-96 (15-Oct) 21 b 60 a 6 b 0.02

1996/97 (8-Nov) 185 a 67 b 12 b 0.01

1997-98** (10-Oct and 11-Nov) 448 a 349 a 34 b 0.0001

* In 1996-97 and 1997-98 gum yields were the average of two tapping dates. ** In 1997-98 gum yields were the average of two tapping dates.

Characteristics of gum produced by Sudan and local provenances

(table V)

Intrinsic viscosity ranged between 16 and 19 mL/g. Only one sample of the local (Cameroon Laf) provenance from Aïssa-Hardé was abnormal, although not exceptional. Similar results were observed on fresh and authenticated samples of Senegalese Ferlo gum, with no fur-ther anomaly (Fenyo, Personal Communication). Similarly, the average molecular masses were typical for gum arabic; the correla-tion between high viscosity and high molecular mass was evident for sample 4. The values of the col-orimetric index were remarkably low. These samples would be clas-sified in the trade as “selected sort” or “hand-picked selected”, i.e. the cleanest, clearest and of greater value. The specific optical rotations were also typical for the gum of A. senegal (Fenyo et al., 1998). The gum collected follow-ing exudation due to insect attacks had similar properties to those obtained by tapping. The Cameroonian gum samples ana-lyzed had classical properties, typical of A. senegal exudates from the Sahelian zone.

Table V.

Intrinsic viscosity, molecular mass, colorimetric index and specific rotation angle of gum samples from Acacia senegal, collected in Northern Cameroon.

Site Makalingay Aïssa Hardé

Origin Gum samples from tapping Gum samples

from insect attacks

Provenance Cameroon Laf Sudan North Sudan Blue Cameroon Laf Cameroon Laf

Kordofan Nile

Viscosity (mL/g) 17.1 18.6 17.7 24.3 16.4

Molecular Mass (g/mol) 375,000 370,000 322,000 1,780,000 718,000

Lovibond colorimetric Index 1.6 1 1.1 1.4 1.2

Specific optical rotation at 25°C -26° - - -27°

-(aqueous solution 1 %)

Tappings carried out for five consecutive years did not seem to exhaust the trees: example of good healing of the bark in Ngong.

Conclusion

Although Northern Cameroon is not as dry as the tradi-tional producing regions of Sudan and Senegal, it proved suit-able for gum arabic production. The local provenance of A. senegal survived and grew well in the different conditions of the study area with 650 to 1250 mm annual rainfall. Between 650 and 800 mm annual rainfall, average gum production was 215 g per tapped tree and could go up to 500 g/tree, i.e. 250 kg/ha/y for a density of 500 trees/ha. A high rainfall dur-ing the wet season preceddur-ing a well-established dry season seemed beneficial for gum production. However, in sites with an annual rainfall higher than 1,000 mm, the gum production was generally lower and uncertain, maybe because of the insufficient water stress encountered in these sites.

One main aim of the study was to determine the opti-mum tapping date in the different sites of Northern Cameroon. As in Sahelian areas, gum exudation occurred after tapping applied when the rain stopped, the air humidity dropped and the trees lost their leaves. Tree tapping applied early in the dry season was more effective than after the seasonal climate transition characterized by a drop in the relative humidity.

On sandy ferruginous soils, annual gum production was more variable than on clay soils as the soil water con-tent was more related to climatic variations on sandy soils. Nevertheless, on a multiannual scale, the levels of produc-tion were comparable for both types of soils. Therefore, numerous locations can be suitable for productive A. sene-gal plantations in Northern Cameroon, provided that soils dry out and trees can defoliate early in the dry season.

The local provenance of A. senegal was the most pro-ductive and should be preferred for plantation. This con-firmed the hypothesis that Sahelian provenances may require a dryer environment for high gum production. Conversely, it showed that gum exudation is not limited to provenances adapted to extreme aridity.

These results on gum production in various pedocli-matic situations pave the way for an in-depth research on the ecophysiological drivers of gum arabic exudation by A. senegal. The longevity and gum production capacity in the long term of A. senegal stands still have to be assessed in different contrasting climate/soil conditions. In perspective, one way to alleviate insufficient water stress in sub-humid areas could be the use of compounds susceptible to have an effect on gummosis and gum exudation, such as Ethephon (2-chloroethylphosphoric acid), which releases ethylene and mimics the physiological stress believed to trigger gummosis.

Acknowledgments

This study was funded by the Institute of Agricultural Research for the Development of Cameroon (IRAD), the Inter-national Center of Cooperation in Agronomic research for the Development (CIRAD) and by French Funds of Assistance and Cooperation, by STABEX and Acaciagum European project (EC FP6 contract 032233, http://incoacaciagum.cirad.fr/).

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